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Cleaning up the exhaust

Diesel engine development is faced with the challenge of reducing the concentration of nitrogen oxides and particulates, while at the same time maintaining fuel consumption at a constant level. Ideally even reducing it. This conflict of aims makes the development task increasingly complex. 

Meeting the Euro standards
The Euro 3 limits currently apply, but as of 2005 commercial vehicles will have to conform to the more stringent Euro 4 standard. And by the time Euro 5 comes into force in 2008, noxious harmful emissions will have to be reduced by between 80 % (hydrocarbons) and 90 % (particulates) over the 1990 baseline.

This requires some form of exhaust after-treatment. To this end, engines are designed either for low nitrogen oxide emission or low particulate emission. In each case the other critical exhaust component is subsequently either removed or reduced. Here are the most reliable methods for after-treatment today:

Exhaust Gas Recirculation
Using this method, known as EGR (Exhaust Gas Recirculation), a portion of the exhaust gas can be cooled and routed back to the induced air. This also smoothens the combustion temperature peaks.

Selective Catalytic Reduction
The selective catalytic reduction (SCR) process can be used downstream of the engine. SCR makes use of ammonia, which is produced on board the vehicle from a non-toxic, easy-to-handle aqueous urea solution, which reduces the nitrogen oxides (NO, NO2) to harmless nitrogen (N2) and water.

NOx storage catalysts
Having NOx storage catalysts installed downstream of the engine also helps reduce nitrogen oxides, but they have the disadvantage of increasing fuel consumption. The NOx is stored on the catalyst during normal engine operation, and is regenerated by adding fuel directly or indirectly to the catalyst periodically.

Filters
As well as measures carried out within the engine for the prevention of particulate formation, particulate filtering can also be carried out downstream of the engine. In this case, the exhaust gas flows through a porous filter structure and particle separation occurs primarily through adhesion. This process can reduce particulate emission by 90%. Volvo has for some years now sold this type of exhaust filter for its 6- and 9-litre engines.
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